1. Field of the Invention
The invention relates to a method for introducing a weft thread in a weaving machine. The invention also relates to a weaving machine for employing a method of this type.
2. Related Art
Weaving machines, more particularly air weaving machines, in which compressed air is fed to a blower, are known. In these machines, one or more main blowers and a number of auxiliary blowers are provided for the purpose of introducing a weft thread into a shed. Weaving machines of this type include a feed device for supplying compressed air to a blower, as known for example from EP 0 442 546 B1, EP 0 879 307 B1 or EP 1 086 265 B1. In the case of weaving machines in which various types of weft threads are introduced into a shed in a pattern, an associated set of main blowers with an associated feed device for compressed air is provided for each of these weft threads. During the introduction of a weft thread, compressed air is fed at a high pressure to the associated main blowers and auxiliary blowers. For the remaining time, compressed air at a lower pressure is fed to these main blowers in order to prevent a weft thread from dropping out of these main blowers. The high pressure is, for example, between 2 and 7 bar, while the lower pressure is, for example, between 0.02 and 1 bar.
Compressed air at a high pressure is supplied, for example by controlling a shut-off valve which is disposed between a buffer vessel containing compressed air at a high pressure and a set of main blowers. The quantity of compressed air supplied at a high pressure can in this case be adjusted with the aid of a motor-controlled throttle valve which is disposed between the buffer vessel and an abovementioned shut-off valve. According to an alternative, this throttle valve can be disposed between an abovementioned shut-off valve and a set of main blowers. A motor-controlled throttle valve of this type comprises, for example, a controllable stepper motor which can be rotated in both directions with a desired number of steps by means of a control unit.
It is known to adjust the quantity of compressed air supplied at a high pressure during weaving as a function of a deviation of a measured insertion parameter. In this context, this quantity can, for example, be adjusted in such a manner that a weft thread which has been introduced reaches the end of the shed at an approximately desired angle position of the weaving machine. According to one option, the deviation, more specifically the difference in time and/or in angle between the instant at which the weft thread reaches the end of the shed and the instant at which the main drive shaft of the weaving machine reaches a defined angle position, is determined. Then, for example, the throttle valve is adjusted in such a manner that the said deviation, more particularly this difference in time and/or angle, vanishes.
Since each weft thread behaves differently during the introduction into a shed, also referred to as the insertion, it is customary to determine a mean deviation for a number of successive insertions of a weft thread of this nature. By way of example, it is possible to determine a mean for the deviations on the basis of a measured insertion parameter, which deviations were determined, for example, for 32 successive insertions. The throttle valve can then be adjusted taking account of this mean deviation which has been determined. It is also known to input a threshold value for the mean deviation, more particularly a mean value which must be exceeded before adjustment of the throttle valve needs to be carried out. If the mean deviation exceeds the said threshold value, the throttle valve is suitably adjusted, for example taking account of this deviation. The abovementioned method has the drawback that the throttle valve sometimes has to be adjusted frequently and unnecessarily or that the throttle valve is not adjusted, is adjusted insufficiently or is adjusted too late. Moreover, the threshold value may be too low for one type of yarn, so that the system will react too quickly, while the said threshold value may be too high for a different type of yarn, causing the system to react too slowly.